Seal for gimbaling and/or fixed rocket engine nozzles, and associated systems and methods

ABSTRACT

Seals for gimbaling and/or fixed rocket engine nozzles, and associated systems and methods are disclosed. A representative rocket propulsion system includes a rocket engine having an exhaust nozzle, a seal plate carried by the exhaust nozzle, and a seal engaged with the seal plate. The seal includes at least one support, multiple pivotable first flaps, carried by the at least one support and positioned to contact the seal plate, and multiple pivotable second flaps, with an individual second flap positioned to shield a corresponding individual first flap. At least one forcing element is operatively coupled to at least one of the individual first flap or the individual second flap, to apply a pivoting force to the at least one of the individual first flap or the individual second flap.

TECHNICAL FIELD

The present disclosure is directed generally to seals for gimbalingand/or fixed rocket engine nozzles, and associated systems and methods.

BACKGROUND

Rockets have been used for many years to launch human and non-humanpayloads into orbit. Such rockets delivered the first humans to spaceand to the moon, and have launched countless satellites into the Earth'sorbit and beyond. Such rockets are used to propel unmanned space probesand more recently to deliver structures, supplies, and personnel to theorbiting international space station.

One continual challenge associated with rocket missions is providingsufficient control authority during all phases of rocket operations. Oneapproach to addressing this challenge is to provide the rocket withgimbaled rocket engines that can change the direction in which theydirect rocket thrust, so as to stabilize and/or redirect the rocket. Onechallenge associated with gimbaled rocket engines is to properly sealthe interface between the engine nozzle and the rocket, despite themovement of the engine nozzle relative to the rocket. Another challengeis protecting the base area of a re-useable rocket that reenters theatmosphere and lands tail first. Aspects of the present disclosure aredirected to addressing this challenge.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially schematic, side elevation view of a representativerocket on which seals in accordance with the present technology can beinstalled.

FIG. 2 is a partially schematic, bottom isometric view of arepresentative first stage of a rocket having both gimbalable andnon-gimbalable engine nozzles, in accordance with embodiments of thepresent technology.

FIG. 3 is a partially schematic, cross-sectional side view of agimbalable engine and nozzle, having a seal arrangement configured inaccordance with embodiments of the present technology.

FIG. 4A is a partially schematic, isometric view of a seal configured tointerface with a gimbalable nozzle in accordance with embodiments of thepresent technology.

FIG. 4B is a partially schematic, enlarged view of a portion of the sealshown in FIG. 4A.

FIG. 5 is a partially schematic, cross-sectional illustration of aportion of a seal configured in accordance with embodiments of thepresent technology.

FIG. 6 is an exploded view of representative components of the sealshown in FIG. 5.

FIGS. 7A-7C illustrate a seal configured in accordance with furtherembodiments of the present technology.

FIGS. 8A-8F illustrate the motion of representative seal flaps duringnormal operation, and during a removal process, in accordance withembodiments of the present technology.

DETAILED DESCRIPTION

Embodiments of the technology disclosed herein are directed generally toseals for gimbaling and/or fixed rocket engine nozzles, and associatedsystems and methods. In particular embodiments, the seal can includemultiple, overlapping (e.g., shingled) flaps that protect the interiorof a reusable rocket stage as it descends through the atmosphere forlanding and reuse. The overlapping seals can include one flap thatprovides a physical seal at the interface between the engine nozzle andthe base heat shield of the rocket, and a second flap that provides heatprotection for the first flap, and provides for shingling. One or moreof the flaps can be biased against the heat shield (either directly, orby acting on an overlapping flap) so as to maintain the integrity of theseal, even as the engine and nozzle move. Such movement may bedeliberate, for example, in the case of a gimbaling engine nozzle,and/or the result of changes in the nozzle dimensions and/or positions,e.g., as the nozzle expands and contracts under thermal loads and/orstructural deformation.

Several details describing structures and processes that are well-knownand often associated with such seals are not set forth in the followingdescription to avoid obscuring other aspects of the disclosure.Moreover, although the following disclosure sets forth severalembodiments, several other embodiments can have differentconfigurations, arrangements, and/or components than those described inthis section. In particular, other embodiments may have additionalelements, and/or may lack one or more of the elements described belowwith reference to FIGS. 1-8F.

FIG. 1 is a partially schematic illustration of a representative system100 configured in accordance with embodiments of the present technology.The system 100 can include a vehicle 101 (e.g., a launch vehicle) havinga single or a multi-stage configuration. In the representativeembodiment shown in FIG. 1, the vehicle 101 includes a first stage 102,a second stage 103, and a payload 104 (shown schematically in FIG. 1)surrounded by a fairing 105. The first stage 102 and the second stage103 operate as boosters to direct the payload 104 into space. In otherembodiments, the vehicle 101 can include a single booster, or more thantwo boosters. In any of these embodiments, at least one of the boosters(e.g., the first stage 102) is configured to be returned to Earth in atail-down configuration, and is then reused on a subsequent launch.

The first stage 102 can include a propulsion system 110 that can in turninclude one or more main engines 111 (positioned within the first stage102). Each main engine 111 can include a corresponding nozzle 112.During launch, the main engines 111 provide the primary force directingthe vehicle 101 upwardly. During a tail-down reentry, the thrustprovided by the main engines 111 provides a braking force on the firststage 102 as it descends and lands in preparation for its next mission.In both cases, thrust is provided along a thrust axis TA, which can beadjusted, as discussed below, to steer or maneuver the vehicle 101.

FIG. 2 is a partially schematic, bottom isometric illustration of thefirst stage 102 shown in FIG. 1, illustrating a base heat shield 113that protects the lower portions of the first stage 102 from heat andaerodynamic forces encountered as the first stage 102 descends throughthe atmosphere. As is also shown in FIG. 2, one or more of the enginenozzles 112 can have a generally fixed or non-gimbalable configuration(four are indicated by reference numbers 112 b), and/or one or more ofthe engines can have a gimbalable configuration (three are indicated byreference numbers 112 a). As used herein, the term “gimbalable” refersto a device that is configured to gimbal in operation. The gimbalableengine nozzles 112 a can pivot about one or more axes so as to vectorthe thrust produced by the corresponding engines and steer the vehicle101 as it descends. The non-gimbalable engine nozzles 112 b can providethrust in a generally fixed direction. In some instances, thenon-gimbalable engine nozzles 112 b are referred to herein as “fixed”nozzles; however, it will be understood that even the “fixed” nozzleschange position with respect to the base heat shield 113, e.g., as aresult of thermal expansion and contraction, and/or structuraldeformation. Accordingly, the seals of the present technology canoperate to seal the gaps between the base heat shield and (a) thegimbalable engine nozzles 112 a, and/or (b) the non-gimbalable enginenozzles 112 b. In general, the same seal can be used for both types ofengine nozzles. However, in some instances, a representative first stage102, such as the one shown in FIG. 2, may include multiple, differenttypes of seals, one for the gimbalable engine nozzles 112 a, and anotherfor the non-gimbalable engine nozzles 112 b.

FIG. 3 is a partially schematic, cross-sectional illustration of agimbalable engine 111 a and associated gimbalable nozzle 112 a. Thenozzle 112 a projects downwardly through a corresponding opening in thebase heat shield 113, and can rotate relative to the first stage 102about one or more axes. For example, the gimbalable nozzle 112 a canrotate about two axes transverse to the thrust axis TA, as indicated byarrows R1 and R2. In addition, the gimbalable nozzle 112 a cantranslate, in a generally vertical direction as indicated by arrow A,and/or in a generally horizontal or lateral direction as indicated byarrow B. This translational movement can apply as well to thenon-gimbalable engine nozzles 112 b shown in FIG. 2.

As is also shown in FIG. 3, the system 100 can include a seal plate 114extending outwardly from the nozzle 112 a. The seal plate 114 can have adownwardly facing sealing surface 115, which can have a curved (e.g.,spherical) shape for a gimbaling nozzle, and a curved, flat, or othersuitable shape for a non-gimbaling nozzle. One or more seals 120 caninclude flaps that contact the sealing surface 115 so as to at leastreduce the penetration of hot gases upwardly into the internal spaces ofthe first stage 102, as the first stage 102 descends. This in turnreduces or eliminates damage to the first stage 102, which in turnreduces the time and cost required to refurbish the first stage 102 fora subsequent flight. Further details of representative seals andassociated advantages, including advantages related to refurbishment,are described below with reference to FIGS. 4A-8F.

FIG. 4A is a partially schematic illustration of a representative seal120 having a circular seal support 121 that carries multiple flaps 140.The flaps contact the sealing surface 115 of the nozzle 112 a, asdiscussed above with reference to FIG. 3. The seal 120 can furtherinclude one or more forcing elements 150 that force or bias the flapsinto contact with the sealing surface, as is described in further detailbelow.

FIG. 4B is an enlarged view of a portion of the seal 120 shown in FIG.4A. As shown in FIG. 4B, the flaps 140 can include a first flap 140 aand a second, underlying flap 140 b. An individual first flap 140 a canbe paired with a corresponding individual second flap 140 b. The edgesof the first and second flaps 140 a, 140 b can be offset from each otherto provide a baffling and/or shingling effect, and thereby reduceleakage at the seal 140.

Each pair of first and second flaps 140 a, 140 b can be driven by acorresponding forcing element 150. The first flap 140 a has a contactsurface 144 that engages with the sealing surface 115 of the enginenozzle (FIG. 3). The second flap 140 b protects the first flap 140 afrom the elevated temperatures and pressures encountered during reentry.For example, in some embodiments, the temperatures behind the bow shockproduced by the descending first stage 102 can reach 4,000° F. or more,and so the second flap 140 b can be formed from, and/or can include, anextreme temperature metal, such as Haynes 230, and/or a carbon-carbonand/or ceramic matrix composite material.

In particular embodiments, the first flap 140 a is generally thickerthan the second flap 140 b, and provides the structural strengthrequired to withstand the pressure produced by the second flap 140 b asthe second flap 140 b pushes against it. For example, the first flap 140a can be formed from, or can include, a material that retains itsstrength at high temperatures, such as Haynes 282 or Inconel 718.Accordingly, the first flap 140 a can provide a mechanical sealing forcewith the sealing surface 115, and can provide support for the secondflap 140 b, while the second flap 140 b provides thermal protection forthe first flap.

In a representative embodiment, the first flap 140 a has a thickness of0.18 inches, and the second flap 140 b has a thickness of 0.08 inches.In other embodiments, one or both of the foregoing flaps can havedifferent dimensions, depending on factors including, but not limitedto, the composition of the flaps, and/or the temperature and/or pressureof the environment in which the flaps operate. In general, the firstflap 140 a may be thicker than the second flap 140 b so as to provide anenhanced structural function, while the second flap provides an enhancedheat shielding function.

In particular embodiments, the thicknesses of both the first and secondflaps 140 a, 140 b are selected such that the flaps have sufficientcapacity to absorb the heat to which they are subjected, without failingto function during the transient high temperature heat excursion thatresults during reentry. Because the temperature capabilities of thematerials may be below the temperature of the surrounding gases, thedesign of the flaps may rely on the relatively short duration of thehigh temperature excursion. For longer duration reentries, one or moreof the flaps can be made from a refractory metal (e.g., amolybdenum/zirconium/niobium alloy), and/or a carbon-carbon material, aceramic material, and/or ceramic matrix composite. Because suchmaterials are typically expensive and/or difficult to manufacture, usingmaterials selected for the expected short-duration reentry can reduceoverall costs.

In a further aspect of an embodiment shown in FIG. 4B (and described ingreater detail with reference to FIG. 5), the forcing element 150 canoperate on the second flap 140 b to drive the first flap 140 a upwardlyinto contact with the corresponding engine nozzle sealing surface. In arepresentative embodiment, the second flap 140 b can include a driveportion 142, for example, a lever arm, that is acted upon by an actuatorrod or piston 170. The actuator rod 170 can be housed in a cylinder orcanister 160, which is in turn attached to a cylinder bracket 162 andcarried by a cylinder support 161. The cylinder support 161 is attachedto the seal support 120. Accordingly, the forcing element 150 can rotateor bias both the second flap 140 b and the first flap 140 a in an upwarddirection. The flaps 140 a, 140 b are rotatably supported by flapbrackets 145.

The forcing element 150 can include one or more springs that bias orforce the second flap 140 b in one or more directions. For example, theforcing element 150 can include a first spring 151 a that biases thesecond flap 140 b in an upward direction. The forcing element 150 canfurther include a second spring 151 b that prevents the second flap 140b from overextending (e.g., over-rotating) in the same direction, forexample, if the seal assembly is positioned on its side rather than inthe horizontal orientation shown in FIG. 4B.

FIG. 5 is a partially cut-away, partially schematic illustration of thearrangement shown in FIG. 4B. The first and second flaps 140 a, 140 bare attached to a flap bracket 145 via one or more flap hinge pins 148.Accordingly, both first and second flaps 140 a, 140 b pivot about thesame axis (or, as shown in the Figures slightly different axes) relativeto the seal support 121. The second flap 140 b, which is positionedbelow the first flap 140 a, includes the drive portion 142, e.g., adriver arm 143, that extends away from the flap hinge pin 148. Theactuator rod 170 is attached to the driver arm 143 via an actuator hingepin 171. The coils of the first spring 151 a are normally spacedslightly apart (when no force is applied to the first spring 151 a), andthe first spring 151 a rests on an actuator base 174 of the actuator rod170. Accordingly, the first spring 151 a has a first spring biasdirection 152 a. If the seal plate 114 moves downwardly against thefirst flap 140 a, the driver arm 143 tends to rotate clockwise, asindicated by arrow R3. The first spring 151 a resists this motion toforce the second flap 140 b upwardly against the first flap 140 a intocontact with the sealing surface 115.

The second spring 151 b can be attached to the actuator base 174 to pushthe actuator rod 170 in an opposite, second spring bias direction 152 b.Accordingly, if the entire seal assembly is rotated counterclockwise,the weight of the first and second flaps may cause them to “flop over”and rotate the driver arm 143 counterclockwise, as indicated by arrowR4, causing the actuator base 174 to separate from the first spring 151a and move toward the bottom of the cylinder 160. The second spring 151b can prevent this from occurring, which facilitates removing andreinstalling the base heat shield and/or nozzle between missions.

FIG. 6 is a partially schematic, exploded view of several of thecomponents described above with reference to FIGS. 4B and 5. The firstflap 140 a includes a flap aperture 146 a that is positioned betweenbracket apertures 147 of a first flap bracket 145 a. A first flap hingepin 148 a passes through the bracket apertures 147 and the flap aperture146 a to allow the first flap 140 a to rotate about the flap hinge axis149. A second flap hinge pin 148 b extends into the corresponding flapaperture 146 b of the second flap 140 b, so that both the first andsecond flap rotate about the same (or approximately the same) flap hingeaxis 149. In other embodiments, a single hinge pin can extend throughboth the first and second flaps 140 a, 140 b.

The second flap 140 b includes the driver arm 143, which is attached tothe actuator rod 170 via an actuator hinge pin 171 that passes throughan actuator aperture 173 at the upper end of the actuator rod 170, andinto a corresponding aperture 139 of the driver arm 143. Accordingly,the actuator rod 170 (which is shown broken into two sections, forpurposes of illustration) can rotate relative to the second flap 140 babout an actuator hinge axis 172, as the actuator rod 170 moves upwardlyand downwardly.

The actuator rod 170 is housed, in part, within the cylinder 160. Thefirst spring 151 a fits around the actuator rod 170 and rests on theactuator base 174. The actuator rod 170 extends outwardly from thecylinder 160 through an aperture 164. The first spring 151 a is capturedwithin the cylinder 160 between the upper end of the cylinder 160, and abase 174 of the actuator rod 170. The second spring 151 b fits between abase 165 of the cylinder 160 and the actuator base 174. A cylinder hingepin 163 pivotably couples the cylinder 160 to the cylinder bracket 162,which is in turn attached to the cylinder support 161 of the sealsupport 121. The corresponding flap brackets 145 a, 145 b are alsoattached to the seal support 121, as indicated by arrows B1 and B2, at aposition above the cylinder bracket 162.

FIGS. 7A-7C illustrate a sealing arrangement in accordance with anotherrepresentative embodiment of the present technology, suitable for both anon-gimbalable nozzle 112 b (FIG. 2) and a gimbalable nozzle 112 a.Referring first to FIG. 7A, the representative nozzle 112 can have aflange 716, which in turn carries a seal plate 714 extending outwardlyfrom the nozzle 112. The seal plate 714 can be generally flat, as shownin FIG. 7A, or curved (e.g., spherical). A seal 720, including a sealsupport 721, can be positioned circumferentially around the nozzle 112to seal the interface between the base heat shield 113 and the sealplate 714.

Referring next to FIG. 7B, the seal 720 can include a first flap 740 apositioned above a second flap 740 b, each of which can pivot about acommon flap hinge pin 748, or two corresponding flap hinge pins. Thesecond flap 740 b can include a driver arm 743 that is connected to anactuator rod 770. The actuator rod 770 extends through an aperture inthe driver arm 743, and connects to the support 721 via an actuatorbracket 775, and an actuator hinge pin 771. Accordingly, the actuatorrod 770 can pivot about the hinge pin 771, as the driver arm 743 pivotsabout the flap hinge pin 748.

The seal 720 can further include a forcing element 750, e.g., a spring751, that bears against a retainer 776, which in turn bears against thedriver arm 743. If the first and second flaps 740 a, 740 b rotateclockwise around the flap hinge pin 748, the spring 751 forces themcounterclockwise, into contact with the corresponding sealing surface715 of the seal plate 714.

FIG. 7C is a partially schematic, exploded view of several of thecomponents shown in FIG. 7B. As shown in FIG. 7C, the first flap 740 aincludes a contact surface 744 that sealably engages with the sealingsurface 715 of the seal plate 714. The second flap 740 b provides heatprotection for the first flap 740 a, and is biased upwardly against thefirst flap 740 a via the spring 751 and actuator rod 770. Each flap 740a, 740 b includes a corresponding aperture 746 a, 746 b to receive theflap hinge pin 748.

FIGS. 8A and 8B illustrate the motion of a representative set of flaps140 (e.g., multiple pairs of first and second flaps 140 a, 140 b), asthe nozzle 112 moves upwardly and downwardly during normal operation.The seal 120 can have a configuration similar to that shown in FIG. 5.Or the seal 120 can have another suitable configuration, for example,that shown in FIGS. 7A-7C. In any of these embodiments, and as shown inFIG. 8A, the nozzle 112 and the seal plate 114 have moved downwardly,and the flaps 140 a, 140 b have followed that motion, maintaining a sealwith the sealing surface 115 of the seal plate 114. In FIG. 8B, thenozzle 112 has moved upwardly, and the flaps 140 have maintained contactwith the sealing surface 115 of the seal plate 114.

The seal 120 can also be configured to accommodate much more significantmotion relative to the nozzle 112, for example, when the base heatshield 113 of the rocket is removed for refurbishment, and/or to accesspropulsion system components and/or other components that are protectedby the base heat shield 113 and the seal 120. For example, referring nowto FIG. 8C, the base heat shield 113, with the seal 120 attached, hasbeen moved downwardly from the seal plate 114 (FIG. 8B), as indicated byarrow D. As the base heat shield 113 continues to move downwardly, theflaps 140 come into contact with the outer surface of the nozzle 112, asis shown in FIG. 8D. Because the flaps 140 are hinged, they can rotateoutwardly as the flared outer surface of the nozzle 112 passes by. Thisis illustrated in FIG. 8E, which shows the flaps 140 rotating outwardly(as indicated by arrow R1) to allow the nozzle 112 to pass. Once theopen end of the nozzle 112 has cleared the flaps 140, as shown in FIG.8F, the flaps 140 return to their neutral position under the biasingforce of the associated springs, as indicated by arrow R2.

When the base heat shield 113 is to be replaced, an optional dilatingtool (not shown) can be used to rotate the flaps 140 outwardly, asindicated by arrow R1 in FIG. 8E, thus allowing the base heat shield 113and the seal 140 to be moved upwardly over the open end of the nozzle112. Once the seal 120 is over the end of the nozzle 112, the dilatingtool can be removed, the flaps 140 can return to their neutralpositions, and the base heat shield 113 can be moved further upwardlyfor attachment to the rocket, reversing the steps described above withreference to FIGS. 8C-8D.

While the discussion above described the base heat shield as being moveddownwardly relative to the nozzle, in at least some embodiments, therocket can be positioned horizontally, and the base heat shield can beremoved and replaced via a lateral motion. As discussed above, thearrangement of springs can both bias the flaps into contact with theassociated sealing surface, and prevent the flaps from over-rotatingfrom their neutral positions, even when the rocket is positionedhorizontally. This arrangement can prevent the flaps 140 frominterfering with the nozzle when the base shield is reinstalled.

An advantage of the foregoing arrangement is that the process ofremoving the base heat shield (for improved access to the nozzle and/orcomponents within the rocket) can be performed without damaging theseal. This approach, alone or together with other elements of thepresent technology, can facilitate repeated rocket launches andlandings, without the need to replace the seal. In addition, the processof refurbishing the seal and/or the base shield is simplified when thesecomponents are removed from the rocket. And while these components mayundergo refurbishment between launches, it is expected that the seal andbase heat shield will remain viable for many launch/landing cycles.

Other features of embodiments of the present technology related torefurbishment and longevity include the hinged nature of the seal, whichallows the seal to be made of metal. Conventional high temperature sealstypically use a protective material that is ablative and/or is otherwisesuitable for one use only, and accordingly must be replaced after eachuse. Embodiments of the present technology avoid this issue. Accordinglyand more generally, a feature of several of the embodiments describedabove with reference to FIGS. 1-8F is that the seal arrangements arereusable. In particular, the seals are designed to withstand the forcesand temperatures associated with multiple launches, landings, andrecovery operations.

Another feature of several of the embodiments described above is thatthey can include forcing elements that in turn include simple springs orother passive elements. An advantage of this feature is that suchelements are less likely to fail and more likely to withstand the rigorsof multiple launch and landing operations.

From the foregoing, it will be appreciated that specific embodiments ofthe disclosed technology have been described herein for purposes ofillustration, but that various modifications may be made withoutdeviating from the technology. For example, in some embodimentsdescribed above, one flap of a flap pair is driven, and in turn drivesthe other flap of the flap pair. The driven flap can be located below anoverlapping flap, or the positions can be reversed. In otherembodiments, both flaps may be driven. As another example, the materialsand material thicknesses may be different than those described above.The system can include biasing mechanisms different than the springarrangements described above. Certain aspects of the technologydescribed in the context of particular embodiments may be combined oreliminated in other embodiments. Further, while advantages associatedwith certain embodiments of the disclosed technology have been describedin the context of those embodiments, other embodiments may also exhibitsuch advantages, and not all embodiments need necessarily exhibit suchadvantages to fall within the scope of the present technology.Accordingly, the disclosure and associated technology can encompassother embodiments not expressly shown or described herein.

As used herein, the terms “generally” and “approximately” refer tovalues or characteristics within a range of ±10% from the stated valueor characteristic, unless otherwise indicated.

1-26. (canceled)
 27. A rocket propulsion system, comprising: a rocketengine having an exhaust nozzle; a seal plate carried by the exhaustnozzle; and a seal engaged with the seal plate, the seal including: atleast one support; multiple pivotable first flaps, with an individualone of the first flaps carried by the at least one support andpositioned to contact the seal plate; multiple pivotable second flaps,with an individual one of the second flaps positioned to shield theindividual one of the first flaps, and wherein the individual one of thefirst flaps is independently pivotable away from the individual one ofthe second flaps; and at least one forcing element operatively coupledto the individual one of the first flaps or the individual one of thesecond flaps, or both, to apply a pivoting force to the individual oneof the first flaps or the individual one of the second flaps.
 28. Thesystem of claim 27 wherein the at least one forcing element isoperatively coupled to the individual one of the second flaps to biasthe individual one of the second flaps into contact with the individualone of the first flaps.
 29. The system of claim 27 wherein theindividual one of the first flaps has a higher rigidity than theindividual one of the second flaps.
 30. The system of claim 27 whereinthe individual one of the first flaps or the individual one of thesecond flaps, or both, includes a carbon-carbon material.
 31. The systemof claim 27 wherein the individual one of the second flaps has a higherheat resistance than the individual one of the first flaps.
 32. Thesystem of claim 27 wherein the individual one of the first flaps or theindividual one of the second flaps, or both, includes a ceramic orceramic matrix composite material.
 33. The system of claim 27 whereinthe at least one forcing element includes a spring coupled to theindividual one of the first flaps or the individual one of the secondflaps via an actuator rod.
 34. The system of claim 33 wherein the springis a first spring, and wherein the system further comprises: a secondspring coupled to the actuator rod, with the first spring positioned tobias the actuator rod in a first axial direction, and the second springpositioned to bias the actuator rod in a second direction opposite thefirst direction; and a canister housing the first spring, the secondspring, and the actuator rod.
 35. The system of claim 27 wherein atleast a portion of the seal plate engaged by the seal is spherical. 36.The system of claim 27 wherein at least a portion of the seal plateengaged by the seal is flat.
 37. The system of claim 27 wherein thenozzle has a thrust axis and is gimbalable relative to the thrust axis.38. The system of claim 27 wherein the nozzle has a thrust axis and isrotationally fixed relative to the thrust axis.
 39. The system of claim38 wherein the nozzle is axially movable along the thrust axis.
 40. Thesystem of claim 27 wherein the nozzle is: positioned to direct thrustalong a thrust axis; axially moveable along the thrust axis; gimbalableabout a first axis transverse to the thrust axis; and gimbalable about asecond axis transverse to the thrust axis and the first axis.
 41. Arocket propulsion system, comprising: a rocket engine having agimbalable exhaust nozzle; a spherical seal plate carried by the exhaustnozzle; and a seal engaged with the seal plate, the seal including: atleast one support; multiple flap pairs arranged circumferentially aroundthe nozzle, each one of the flap pairs including: a first pivotable flappositioned to contact the seal plate; a second pivotable flap positionedto shield the first pivotable flap, wherein the first and secondpivotable flaps are independently pivotable, and wherein the firstpivotable flap of one of the multiple flap pairs overlaps the secondpivotable flap of a neighboring one of the multiple flap pairs; anactuator rod coupled to the second pivotable flap; a first springcoupled to the actuator rod to bias the second pivotable flap in a firstrotary direction; a second spring coupled to the actuator rod to biasthe second pivotable flap in a second rotary direction opposite thefirst rotary direction; and at least one support carrying the first andsecond pivotable flaps.
 42. The system of claim 41 wherein the firstspring, the second spring, and the actuator rod are housed, at least inpart, in a cylinder, and wherein the cylinder is pivotably carried bythe at least one support.
 43. The system of claim 41 wherein multiplespring pairs are positioned circumferentially around the nozzle.
 44. Aseal for a rocket engine, comprising: at least one support; multiple,pivotable first flaps, each having a sealing surface, with an individualone of the first flaps carried by the at least one support; multiple,pivotable second flaps, with an individual one of the second flaps (i)positioned to shield the individual one of the first flaps, and (ii)independently pivotable relative to the individual one of the firstflaps; and at least one forcing element operatively coupled to theindividual one of the first flaps or the individual one of the secondflaps, or both, to apply a pivoting force to the individual one of thefirst flaps or the individual one of the second flaps.
 45. The seal ofclaim 44 wherein the at least one forcing element includes a springcoupled to the individual one of the first flaps or the individual oneof the second flaps via an actuator rod.
 46. The seal of claim 45wherein the spring is a first spring, and wherein the system furthercomprises: a second spring coupled to the actuator rod, with the firstspring positioned to bias the actuator rod in a first axial direction,and the second spring positioned to restrict motion of the actuator rodin a second direction opposite the first direction; and a canisterhousing the first spring, the second spring, and the actuator rod.